Technical Field
The present disclosure relates to methods for requesting uplink resources by a user equipment in a communication system. The present disclosure is also providing the user equipment for participating in the methods described herein.
Description of the Related Art
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system, which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport, as well as high capacity voice support, for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Release 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed, including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE Release 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically, to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at the time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME, and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN), and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling, and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain into so-called subframes. In 3GPP LTE, each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a given number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers, as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block (PRB) is defined as NsymbDL consecutive OFDM symbols in the time domain (e.g., 7 OFDM symbols) and NscRB consecutive subcarriers in the frequency domain, as exemplified in FIG. 4 (e.g., 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see, for example, 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following, the time-frequency resources equivalent to the same NscRB consecutive subcarriers spanning a full subframe are called a “resource block pair”, or equivalent “RB pair” or “PRB pair”.
The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Similar assumptions for the component carrier structure also apply to later releases.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g., to fulfill the requirements on IMT-Advanced.
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz, even though these cells in LTE may be in different frequency bands.
All component carriers can be configured to be LTE Release 8/9 compatible, at least when the bandwidth of a component carrier does not exceed the supported bandwidth of an LTE Release 8/9 cell. Not all component carriers aggregated by a user equipment may necessarily be Release 8/9 compatible. Existing mechanism (e.g., barring) may be used to avoid Release 8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells), depending on its capabilities. An LTE-A Release 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Release 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Release 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers, with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may currently not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not provide the same coverage.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g., TAI), similarly as in LTE Release 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC).
The characteristics of the downlink and uplink PCell are:                For each SCell, the usage of uplink resources by the UE in addition to the downlink resources is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only);        The downlink PCell cannot be deactivated, unlike SCells;        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF;        Non-access stratum information is taken from the downlink PCell;        PCell can only be changed with handover procedure (i.e., with security key change and RACH procedure);        PCell is used for transmission of PUCCH;        The uplink PCell is used for transmission of Layer 1 uplink control information; and        From a UE viewpoint, each uplink resource only belongs to one serving cell.        
The configuration and reconfiguration, as well as the addition and removal, of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Release 8/9 for handover).
When a user equipment is configured with carrier aggregation there is at least one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. The same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled on multiple component carriers simultaneously, but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking, established by RRC signaling, between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data, thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g., a subframe of 0.5 ms, onto which coded information bits are mapped. It should be noted that a subframe, also referred to as a transmission time interval (TTI), is the smallest time interval for user data transmission. It is, however, possible to assign a frequency resource BW grant over a longer time period than one TTI to a user by concatenation of subframes.
UL Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e., controlled by eNB, and contention-based access.
In case of scheduled access, the UE is allocated a certain frequency resource for a certain time (i.e., a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, UEs can transmit without first being scheduled. One scenario where UE is making a contention-based access is, for example, the random access, i.e., when UE is performing initial access to a cell, or for requesting uplink resources.
For the scheduled access, Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically, the scheduler determines:                which UE(s) is (are) allowed to transmit;        which physical channel resources (frequency); and        Transport format (Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission.        
The allocation information is signaled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons, this channel is called uplink grant channel in the following. A scheduling grant message contains information about which part of the frequency band the UE is allowed to use, the validity period of the grant, and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one subframe. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e., there are no “per UE per RB” grants). Therefore the UE needs to distribute the allocated resources among the radio bearers according to some rules. Unlike in HSUPA, there is no UE based transport format selection. The eNB decides the transport format based on some information, e.g., reported scheduling information and QoS info, and the UE has to follow the selected transport format. In HSUPA, the Node B assigns the maximum uplink resource, and the UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of Service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management:                Starvation of low priority services should be avoided;        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme;        The UL reporting should allow fine granular buffer reports (e.g., per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent;        It should be possible to make clear QoS differentiation between services of different users; and        It should be possible to provide a minimum bit rate per radio bearer.        
As can be seen from above list, one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other transmission-related information (e.g., HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length can be a multiple of the subframes. The TTI length may be fixed in a service area for all users, may be different for different users, or may even be dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI. Without loss of generality, the following assumes that a TTI is equivalent to one subframe.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which in most cases includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH. Furthermore, Release 11 introduced an EPDCCH that fulfills basically the same function as the PDCCH, i.e., conveys L1/L2 control signaling, even though the detailed transmission methods are different from the PDCCH. Further details can be found particularly in the current versions of 3GPP TS 36.211 and 36.213, incorporated herein by reference. Consequently, most items outlined in the background and the embodiments apply to PDCCH as well as EPDCCH, or other means of conveying L1/L2 control signals, unless specifically noted.
Generally, the information sent on the L1/L2 control signaling for assigning uplink or downlink radio resources (particularly LTE(-A) Release 10) can be categorized to the following items:                User identity, indicating the user that is allocated. This is typically included in the checksum by masking the CRC with the user identity;        Resource allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Alternatively this information is termed resource block assignment (RBA). Note that the number of RBs on which a user is allocated can be dynamic;        Carrier indicator, which is used if a control channel transmitted on a first carrier assigns resources that concern a second carrier, i.e., resources on a second carrier or resources related to a second carrier (cross carrier scheduling);        Modulation and coding scheme that determines the employed modulation scheme and coding rate;        HARQ information, such as a new data indicator (NDI) and/or a redundancy version (RV) that is particularly useful in retransmissions of data packets or parts thereof;        Power control commands to adjust the transmit power of the assigned uplink data or control information transmission;        Reference signal information such as the applied cyclic shift and/or orthogonal cover code index, which are to be employed for transmission or reception of reference signals related to the assignment;        Uplink or downlink assignment index that is used to identify an order of assignments, which is particularly useful in TDD systems;        Hopping information, e.g., an indication of whether and how to apply resource hopping in order to increase the frequency diversity;        CSI request, which is used to trigger the transmission of channel state information in an assigned resource; and        Multi-cluster information, which is a flag used to indicate and control whether the transmission occurs in a single cluster (contiguous set of RBs) or in multiple clusters (at least two non-contiguous sets of contiguous RBs). Multi-cluster allocation has been introduced by 3GPP LTE-(A) Release 10.        
It is to be noted that the above listing is non-exhaustive, and not all mentioned information items need to be present in each PDCCH transmission, depending on the DCI format that is used.
Downlink control information occurs in several formats that differ in overall size and also in the information contained in their fields. The different DCI formats that are currently defined for LTE are as follows, and are described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (current version v12.3.0 available at http://www.3gpp.org and incorporated herein by reference). In addition, for further information regarding the DCI formats and the particular information that is transmitted in the DCI, please refer to the aforementioned technical standard or to “LTE—The UMTS Long Term Evolution—From Theory to Practice”, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.
Additional formats may be defined in the future.
DRX (Discontinuous Reception)
DRX functionality can be configured for RRC_IDLE, in which case the UE uses either the specific or default DRX value (defaultPagingCycle); the default is broadcasted in the System Information, and can have values of 32, 64, 128 and 256 radio frames. If both specific and default values are available, the shorter value of the two is chosen by the UE. The UE needs to wake up for one paging occasion per DRX cycle, the paging occasion being one subframe.
DRX functionality can be also configured for an “RRC_CONNECTED” UE, so that it does not always need to monitor the downlink channels. In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321 v12.4.0 Chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e., the On-Duration periods at which the mobile node is active, and the periods where the mobile node is in a DRX mode:                On-Duration: duration in downlink subframes that the user equipment, after waking up from DRX, receives and monitors the PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200];        DRX inactivity timer: duration in downlink subframes that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e., not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560];        DRX Retransmission timer: specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33];        DRX short cycle: specifies the periodic repetition of the On-Duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640];        DRX short cycle timer: specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX inactivity timer has expired. This parameter is optional. [1-16 subframes]; and        Long DRX Cycle Start offset: specifies the periodic repetition of the On-Duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when On-Duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0—subframe length of chosen cycle]].        
The total duration that the UE is awake is called “Active Time”. The Active Time includes the On-Duration of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired, and the time UE is performing continuous reception while waiting for a downlink retransmission after one HARQ RTT. Similarly, for the uplink the UE is awake at the subframes where uplink retransmission grants can be received, i.e., every 8ms after initial uplink transmission until maximum number of retransmissions is reached. Based on the above, the minimum Active Time is of fixed length equal to On-Duration, and the maximum is variable depending on, e.g., the PDCCH activity.
The “DRX period” is the duration of downlink subframes during which a UE can skip reception of downlink channels for battery saving purposes. The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in DRX (i.e., is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed some other time, during the DRX opportunity of time.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. For example, in case of a web browsing service, it is usually a waste of resources for a UE to continuously receive downlink channels while the user is reading a downloaded web page. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, a short DRX period is better for faster response when data transfer is resumed—for example, when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE; the short DRX cycle is optional, i.e., only the long DRX cycle is used. The transition between the short DRX cycle, the long DRX cycle, and continuous reception is controlled either by a timer or by explicit commands from the eNodeB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle. If data arrives at the eNodeB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next On-Duration time, and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNodeB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
During the Active Time, the UE monitors PDCCH, reports SRS (Sounding Reference Signal) as configured and reports CQI (Channel Quality Information)/PMI (Precoding Matrix Indicator)/RI (Rank Indicator)/PTI (Precoder Type Indication) on PUCCH. When UE is not in Active Time, type-O-triggered SRS and CQI/PMI/RI/PTI on PUCCH may not be reported. If CQI masking is set up for the UE, the reporting of CQI/PMI/RI/PTI on PUCCH is limited to On-Duration.
Available DRX values are controlled by the network and start from non-DRX, going up to x seconds. Value x may be as long as the paging DRX used in RRC_IDLE. Measurement requirements and reporting criteria can differ according to the length of the DRX interval; i.e., long DRX intervals may have more relaxed requirements (for more details see further below). When DRX is configured, periodic CQI reports can only be sent by the UE during “Active Time”. RRC can further restrict periodic CQI reports so that they are only sent during the On-Duration.
FIG. 8 discloses an example of DRX. The UE checks for scheduling messages (indicated by its C-RNTI, cell radio network temporary identity, on the PDCCH) during the “On-Duration” period, which is the same for the long DRX cycle and the short DRX cycle. When a scheduling message is received during an “On-Duration”, the UE starts an “inactivity timer” and monitors the PDCCH in every subframe while the inactivity timer is running. During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling message is received while the inactivity timer is running, the UE restarts the inactivity timer, and when it expires the UE moves into a short DRX cycle and starts a “short DRX cycle timer”. The short DRX cycle may also be initiated by means of a MAC Control Element. When the short DRX cycle timer expires, the UE moves into a long DRX cycle.
In addition to this DRX behavior, a ‘HARQ Round Trip Time (RTT) timer’ is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least ‘HARQ RTT’ subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern.
Buffer Status/Scheduling Request Reporting
The usual mode of scheduling is dynamic scheduling, by means of downlink assignment messages for the allocation of downlink transmission resources and uplink grant messages for the allocation of uplink transmission resources; these are usually valid for specific single subframes. They are transmitted on the PDCCH using C-RNTI of the UE as already mentioned before. Dynamic scheduling is efficient for services types, in which the traffic is bursty and dynamic in rate, such as TCP.
In addition to the dynamic scheduling, a persistent scheduling is defined, which enables radio resources to be semi-statically configured and allocated to a UE for a longer time period than one subframe, thus avoiding the need for specific downlink assignment messages or uplink grant messages over the PDCCH for each subframe. Persistent scheduling is useful for services such as VoIP for which the data packets are small, periodic and semi-static in size. Thus, the overhead of the PDCCH is significantly reduced compared to the case of dynamic scheduling.
Buffer status reports (BSR) from the UE to the eNodeB are used to assist the eNodeB in allocating uplink resources, i.e., uplink scheduling. For the downlink case, the eNB scheduler is obviously aware of the amount of data to be delivered to each UE; however, for the uplink direction, since scheduling decisions are done at the eNB and the buffer for the data is in the UE, BSRs have to be sent from the UE to the eNB in order to indicate the amount of data that needs to be transmitted over the UL-SCH.
Buffer Status Report MAC control elements for LTE consist of either: a long BSR (with four buffer size fields corresponding to LCG IDs #0-3) or a short BSR (with one LCG ID field and one corresponding buffer size field). The buffer size field indicates the total amount of data available across all logical channels of a logical channel group, and is indicated in number of bytes encoded as an index of different buffer size levels (see also 3GPP TS 36.321 v 12.4.0 Chapter 6.1.3.1, incorporated herewith by reference).
Which one of either the short or the long BSR is transmitted by the UE depends on the available transmission resources in a transport block, on how many groups of logical channels have non-empty buffers and on whether a specific event is triggered at the UE. The long BSR reports the amount of data for four logical channel groups, whereas the short BSR indicates the amount of data buffered for only the highest logical channel group.
The reason for introducing the logical channel group concept is that even though the UE may have more than four logical channels configured, reporting the buffer status for each individual logical channel would cause too much signaling overhead. Therefore, the eNB assigns each logical channel to a logical channel group; preferably, logical channels with same/similar QoS requirements should be allocated within the same logical channel group.
In order to be robust against transmission failures, there is a BSR retransmission mechanism defined for LTE; the retransmission BSR timer is started or restarted whenever an uplink grant is restarted; if no uplink grant is received before the retransmission BSR timer expires, another BSR is triggered by the UE.
A BSR is triggered for events, such as:                Whenever data arrives for a logical channel, which has a higher priority than the logical channels whose buffer is non-empty (i.e., whose buffer previously contained data);        Whenever data becomes available for any logical channel, when there was previously no data available for transmission (i.e., all buffers previously empty);        Whenever the retransmission BSR time expires;        Whenever periodic BSR reporting is due, i.e., periodic BSR timer expires; and        Whenever there is a spare space in a transport block which can accommodate a BSR.        
More detailed information with regard to BSR and in particular the triggering of same is explained in 3GPP TS 36.321 v12.4.0 in subclause 5.4.5 incorporated herewith by reference.
If the UE has no uplink resources allocated for including a BSR in the transport block when a BSR is triggered, the UE sends a scheduling request (SR) to the eNodeB so as to be allocated with uplink resources to transmit the BSR. Either a single-bit scheduling request is sent over the PUCCH (dedicated scheduling request, D-SR), or the random access procedure (RACH) is performed to request an allocation of an uplink radio resource for sending a BSR.
However, for sake of completion it should be noted that the UE will not trigger the transmission of the scheduling request for the case that a periodic BSR is to be transmitted.
Furthermore, an enhancement to the SR transmission has been introduced for the specific scheduling mode where resources are persistently allocated with a defined periodicity in order to save L1/L2 control signaling overhead for transmission grants, which is referred to as semi-persistent scheduling (SPS). One example for a service, which has been mainly considered for semi-persistent scheduling, is VoIP. Every 20 ms a VoIP packet is generated at the Codec during a talking spurt. Therefore, the eNodeB can allocate uplink or respectively downlink resources persistently every 20 ms, which could then be used for the transmission of VoIP packets. In general, SPS is beneficial for services with predictable traffic behavior, i.e., constant bit rate, packet arrival time is periodic. For the case where SPS is configured for the uplink direction, the eNodeB can turn off SR triggering/transmission for certain configured logical channels, i.e., BSR triggering due to data arrival on those specific configured logical channels will not trigger an SR. The motivation for such kind of enhancements is that sending a scheduling request for those logical channels which will use the semi-persistently allocated resources (logical channels which carry VoIP packets) is of no value for eNB scheduling and hence should be avoided.
FIG. 8 illustrates in an exemplary way the UE behavior relating to BSR/SR when data arrives in the transmission buffer of the UE (UE Tx buffer). For explanatory purposes the following somewhat simplified scenario is assumed. Only one transmission buffer and one logical channel of a UE is considered. The transmission buffer is assumed to be empty at the beginning, i.e., no data is stored in the transmission buffer. Furthermore, the UE shall not have uplink resources to transmit a buffer status report to the eNodeB. However, the UE shall have semi-statically-allocated resources (e.g., allocated by means of RRC signaling) available in the PUCCH for transmitting a scheduling request (may also be referred to as dedicated scheduling request, D-SR), when necessary; i.e., performing a RACH procedure to send the scheduling request is thus not necessary, which simplifies the illustration.
This simplified scenario shall not be understood as limiting. Similar considerations apply to scenarios additionally including transmission buffers of other logical channels, as well as scenarios where the transmission buffers of a logical channel group, which consists of the transmission buffers of several logical channels, are considered together. Also, the transmission buffer(s) need not be empty; in said case, however, the new data (i.e., the data that currently arrives) entering the transmission buffer of the UE shall have a higher priority than the data already previously stored in the transmission buffer. Moreover, instead of using the allocated resources of the PUCCH for transmitting the SR, the UE might have to perform a RACH procedure to transmit the scheduling request in case no such D-SR uplink resources are available; in the present application, the expressions “scheduling request occasion” (also “scheduling request transmission occasion”) are used to refer to both, i.e., to a dedicated SR and to the RACH procedure.
When new data arrives in the transmission buffer of the UE at time t1, the UE has to first request uplink resources for transmission of the data since no appropriate uplink resources are momentarily available in said respect. Thus, according to the standard trigger condition as explained above, a BSR is triggered in the UE, and in view of the lack of uplink resources for transmitting even the BSR, a scheduling request is triggered in the UE for transmission.
The UE uses the (periodically) allocated PUCCH resources (or RACH procedure not shown in FIG. 8) to transmit the scheduling request to the eNodeB so as to request the eNodeB to allocate uplink resources to the UE. Accordingly, the eNodeB allocates some UL-SCH resources to the UE. Depending, e.g., on the current resource usage in the uplink, the eNodeB may allocate less or more uplink resources to the UE in response to the SR, and will transmit a corresponding uplink grant via the PDCCH.
Upon receiving the uplink grant message, the UE may or may not transmit data in addition to the BSR, depending on the amount of allocated PUSCH resources. When generating the BSR, this is considered by the UE, such that the BSR indicates the amount of data in the transmission buffer after transmitting the BSR and possibly data of the transmission buffer.
Thus, the UE will transmit over the PUSCH only the BSR, or may also include some data of the UE transmission buffer. In FIG. 8, in the first signaling exchange, it is assumed that the UE can transmit all data of the transmission buffer to the eNodeB using the uplink resources assigned by the eNodeB in response to the SR. Correspondingly, the BSR informs the eNodeB about basically an empty transmission buffer, such that no further uplink grant is necessary to be allocated.
However, usually more than one uplink transmission will be necessary in order to empty the transmission buffer, as is also illustrated in FIG. 8 in connection with new data arriving at time t2. In this case, the amount of data is larger than the data becoming available in the transmission buffer at time t1. However, the above procedure basically repeats itself as illustrated, with the exception that the PUSCH transmission, including the BSR and the data, does not suffice to empty the transmission buffer. Correspondingly, the BSR generated by the UE at the time of transmission informs the eNodeB about the remaining data in the transmission buffer. The eNodeB thus will allocate to the UE further uplink resources in correspondence with the remaining data in the transmission buffer. A further uplink grant message is transmitted by the eNodeB to the UE, which in turn can then use the newly-assigned uplink resources to transmit the remaining data, and thus empty its transmission buffer.
Each time new data arrives in the buffer, this basic procedure (or similar) will be repeated.
There has been already in the past discussion during various work items like DDA (Diverse Data Application) with the main goal to identify and specify mechanisms at the radio access network level that enable enhancing the ability of the LTE to handle diverse traffic profiles. In particular, one major goal is to reduce the power usage of the terminals in order to extend the battery life. Also, for certain MTC (machine-type-communication) devices, low power consumption might be a very critical requirement. Hence, some enhancements for minimizing the power consumption were already discussed for services with a specific traffic profile.
This is particularly important for certain types of traffic where the frequency of scheduling request transmissions is quite high due to many small uplink transmissions; for example, for normal interactive TCP traffic where there is a need to send requests for frequent TCP ACKs, or for conversion of speech and/or video traffic which has frequently occurring uplink transmissions. The frequent transmission of scheduling requests will obviously increase the power consumption of the user equipment and also increases the load on the scheduling request channel, as well as the number of PDCCH grants in response to the received scheduling requests.
Very recently, an improvement to the above-discussed BSR/SR reporting mechanism was introduced in the 3GPP standardization. Basically, so as to reduce the power consumption of the user equipment, the triggering of a scheduling request is delayed for some predefined time. By delaying the triggering of the scheduling request, the number of scheduling requests can be reduced, since, due to the delay, the corresponding BSR may take into account additional data arriving in the transmission buffer(s) after the initial new-data trigger until the delay expires.
For said purpose, it was agreed to introduce a dedicated timer logicalChannelSR-ProhibitTimer and configure a corresponding timer value logicalChannelSR-ProhibitTimer-r12 with seven different timer values as 20, 40, 64, 1238, 512, 1024, and 2560 subframes (ms); see information element MAC-MainConfig in 3GPP TS 36.331 v12.4.1 incorporated herewith by reference. As can be appreciated from 3GPP TS 36.321 v12.4.0, subclause 5.4.5 “Buffer Status Reporting”, incorporated herein by reference, the triggering of a scheduling request (if a Regular BSR has been triggered) is additionally dependent on that the logicalChannelSR-ProhibitTimer is not running.
Obviously, for some kind of data, the additional delay may be tolerated, while for other traffic scenarios it is important to send the scheduling request/buffer status report as quickly as possible. Consequently, the delay feature is configured specifically for particular logical channels, e.g., those which can tolerate some delay; put differently, the delay feature is thus logical channel specific. Therefore, the eNodeB can configure for each logical channel whether a delay for the triggering of the scheduling request shall be applied or not. This can be done by way of the information element LogicalChannelConfig and the corresponding optional Boolean variable logicalChannelSR-Prohibit-r12; see information element LogicalChannelConfig in 3GPP TS 36.331 v12.4.1 incorporated herewith by reference.
The effect of this new delay feature will be illustrated in connection with FIG. 9, which assumes a similar scenario as for FIG. 8, and illustrates the triggering of a BSR and SR. Only one transmission buffer and one logical channel of a UE is considered, for simplicity. The transmission buffer is assumed to be empty at the beginning, i.e., no data is stored in the transmission buffer. Furthermore, the UE shall not have sufficient uplink resources to transmit a buffer status report to the eNodeB. However, the UE shall have semi-statically-allocated resources (e.g., allocated by means of RRC signaling) available in the PUCCH for transmitting a scheduling request (may also be referred to as dedicated scheduling request, D-SR), when necessary; i.e., performing a RACH procedure to send the scheduling request is thus not necessary, which simplifies the illustration. Furthermore, it is assumed that the one logical channel of the scenario is configured for the SR delay function, as explained above.
When new data arrives in the transmission buffer of the UE at time t1, in the absence of corresponding uplink resources, a buffer status report cannot be transmitted immediately in the uplink, and thus the UE has to first request uplink resources for transmission of the buffer status report by transmission of a scheduling request. Since the data that triggered the BSR is associated with a logical channel for which the scheduling request (trigger) delay feature was configured, instead of immediately triggering the transmission of the scheduling request, the triggering is delayed by a fixed amount of time according to the preconfigured prohibition timer value, as explained above. In other words, the scheduling request prohibition timer is started when the (Regular) BSR has been triggered, and, while the scheduling request prohibition timer is running, the triggering of the scheduling request is not executed, i.e., it is delayed. On the other hand, after the scheduling request prohibition timer expires, the scheduling request is finally triggered, and the UE can transmit the scheduling request to the eNodeB in the next scheduling request occasion (i.e., by using the periodically allocated PUCCH resources shown in FIG. 9, or the RACH procedure not shown in FIG. 9).
As with the procedure explained before in connection with FIG. 8, the eNodeB allocates UL-SCH resources to the UE, and transmits a corresponding uplink grant via the PDCCH. Upon receiving the uplink grant message, the UE may or may not transmit data in addition to the BSR, depending on the amount of allocated PUSCH resources. In the particular example assumed for FIG. 9, the UE can transmit all data of the transmission buffer to the eNodeB using the uplink resources assigned by the eNodeB in response to the scheduling request. Correspondingly, the buffer status report informs the eNodeB about basically an empty transmission buffer such that no further uplink grant is necessary to be allocated.
FIG. 9 additionally illustrates the case where first data arrives in the transmission buffer at time t2, and additional data arrives in the same transmission buffer at time t3. The arrival of new data in the empty transmission buffer at time t2 triggers the buffer status report, which, however, at basically the same time, also would trigger the transmission of a scheduling request (since no corresponding uplink resources are available for immediately transmitting the buffer status report). Again, the scheduling request trigger is delayed by use of the corresponding prohibition timer, started upon arrival of the new data in the transmission buffer (i.e., the triggering of the BSR). The additional data arriving at time t3 is taking into account by the user equipment when generating the BSR to be transmitted in the uplink to the eNodeB, which would not be the case without the scheduling request delay. Consequently, the transmission of a further scheduling request is avoided, and power consumption can be reduced.